1. Field of the Invention
The invention quite generally relates to projection objectives of microlithographic projection exposure apparatus, which have at least one mirror with a mirror carrier and a reflective coating applied thereon. In particular, the invention relates to EUV projection objectives that use projection light with a wavelength in the extreme ultraviolet spectral range (EUV).
2. Description of Related Art
Microlithographic projection exposure apparatus, as used for manufacturing integrated electrical circuits and other micro-structured components, reproduce structures, which are contained in a reticle, generally reduced on a photo-sensitive layer that can be applied on a silicon wafer for example.
One of the main aims in the development of projection exposure apparatus consists of being able to lithographically define structures with increasingly smaller dimensions on the photo-sensitive layer. The production of particularly small structural sizes requires a low resolution of the projection objectives. Since the resolution of the projection objectives is proportional to the wavelength of the projection light, successive generations of such projection exposure apparatus use projection light with ever shorter wavelengths. Future projection exposure apparatus will presumably use projection light with a wavelength which lies in the extreme ultraviolet spectral range (EUV). To be considered here are in particular wavelengths between 1 nm and 30 nm, and more particularly the wavelength 13.5 nm.
When using projection light with such short wavelengths, no sufficiently transparent materials are available for manufacturing lenses and other refractive optical elements. Therefore EUV projection objectives are substantially composed of mirrors. The mirrors consist of a mirror support, which for example can be manufactured from a glass blank and which has a surface that is manufactured with high precision. Since the mirror support for the projection light is almost 100% absorbent and therefore does not reflect any projection light, a reflective coating with a reflectivity in the order of approximately 60% to 70%, is applied on the surface that is exposed to the projection light.
Coated mirrors however are also used in projection objectives that are designed for longer wavelengths. In order to avoid chromatic aberrations, for example, projection objectives that are suitable for the wavelength 157 nm frequently have a catadioptric structure. This means that they also contain at least one mirror as well as refractive optical elements such as lenses.
Due to the small size of the structures to be reproduced, high demands are made on the reproduction properties of the projection objective. Aberrations can therefore only be tolerated to an absolute minimum.
Generally aberrations are divided into the following two categories. On the one hand there are aberrations which result from the design of the projection objective, i.e. from the specification of the dimensions, materials and distances of the optical elements contained in the projection objective. These design errors will not be regarded in the following.
On the other hand there are aberrations that are due to manufacturing or material defects and generally can only be corrected properly on the completed projection objective. In the case of mirrors for projection objectives, form errors represent the most important manufacturing defects. The term “form error” relates quite generally to deviations of an actual optical surface from its desired form.
In order to correct such manufacturing- or material-related aberrations, U.S. Pat. No. 6,266,389 B1 proposes, in connection with an EUV projection objective, that the finally assembled and adjusted projection objective is measured and then the surface of one or several mirrors is reprocessed in such a way that certain imaging properties of the projection objective are improved. Here, the reprocessing takes place in such a manner that material is removed locally from the coating of the mirrors and/or material is applied locally by laminating onto the coating.
However, it has been shown that the reflectivity of the coating as a result of the local reprocessing substantially changes—and to be more precise—generally decreases. Although the known manufacturing process allows to reduce wavefront errors, the uniformity of the light intensity distribution in the image plane of the projection objective may, to an intolerable extent, deteriorate due to the locally altered reflectivities of individual mirrors.